I found them! I actually found them a while a go, but never got around to telling anyone. I figured I'd post it like this so it could be easily saved/shared/reposted without losing context:
(http://i.imgur.com/5p0oQ.png)
It kind of exited me to find out Donald Knuth had actually commented on this (He's math-famous, mostly for writing The Art of Computer Programming and inventing/writing Tex and Metafont).
I thought I also scanned the question and more context for the rock engravings, but I can't find them. If anyone is really interested and unable to access a library with Scientific American archives I would be willing to lend the book again and scan more, although it might be better to have someone with access to a color scanner do it.
Wow, great find, Sjaako!
BTW these images are not in the scans of the 3rd edition we have.
Yeah, August 1969 is just shortly before the PD 4th edition was published, so that makes sense.